4.1 Water systems

Sunlight supplies energy for evaporation; condensation releases latent heat; gravity drives drainage, runoff and flow downhill.

Separate phase change from movement. Solar energy changes liquid to vapour, while gravity moves precipitation through soil, rivers and groundwater toward lower elevation.

Sun warms a lake until water evaporates; after rain, gravity pulls runoff into a stream.

Gravity, not sunlight; sunlight supplied the earlier evaporation energy.

Do not assign one driver to the whole cycle—different arrows have different causes.

Draw stores as boxes and flows as labelled arrows from source to destination, then state the system boundary.

At global scale, water matter is approximately closed: it changes state and location. Energy crosses the boundary as solar input and heat loss, so the system is open to energy.

A diagram can show ocean → evaporation → atmosphere → precipitation → river → ocean, with each arrow named.

The boundary determines which inputs and outputs count; a catchment is open even when the global cycle is nearly closed.

A store is an amount, not a process; flows need direction and often a rate.

Oceans hold about 96.5% of Earth’s water; ice and groundwater are the next large stores, while rivers, lakes, air and organisms are tiny fractions.

Use the order of magnitude to interpret access: global abundance does not mean freshwater is easy to reach, clean or renew quickly.

A lake may be vital locally but still contain only a minute share of global water compared with the ocean.

Most water is saline or locked in slow stores; accessible freshwater is a small, uneven fraction.

A percentage of global water is not a measure of local availability or sustainable supply.

Transformations change water state; transfers move water, and infiltration is entry into soil while percolation is movement through it.

Evaporation, condensation, freezing, melting and sublimation change state. Advection moves vapour or droplets horizontally; precipitation, runoff, streamflow and groundwater flow move water between locations.

Rain infiltrates the surface, then percolates through porous soil toward groundwater.

Advection; it is a transfer, not condensation or precipitation.

Infiltration and percolation are sequential but not synonyms.

Land use changes interception, evapotranspiration, infiltration and drainage, which can alter peak flow, recharge and flood risk.

Deforestation can reduce canopy storage; compaction and urban surfaces reduce infiltration and speed runoff. Irrigation may raise evapotranspiration or runoff depending on soil, rate and drainage.

Replacing permeable ground with roads shortens the time to peak flow and can increase flash flooding after the same rainfall.

Show less infiltration, faster runoff and a changed hydrograph—not paving and flood timing alone.

One land-use change can affect several flows; do not assume every deforestation site has the same direction or size of effect.

For a defined body and time period, change in storage = inputs − outputs; sustainable extraction must leave the ecological and storage balance viable.

At steady state, inputs equal natural outputs plus harvest. Add seasonal variation, ecological flows, uncertainty and slow recharge before setting a real quota.

A lake receives 180 units and loses 150, so the arithmetic maximum extraction is 30 units for that period—but a safe quota may be lower.

It ignores drought, ecosystem needs and future recharge; arithmetic balance is a ceiling, not a guarantee.

Never quote a budget without its boundary, time period and units.